The Smallest Black Holes and Biggest Neutron Stars

Title: Observation of Gravitational Waves from the Coalescence of a 2.5 – 4.5 M⊙ Compact Object and a Neutron Star

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration

Status: published on ArXiv, Zenodo, & LIGO DCC (all open access)

September 14th, 2015 may be remembered as one of the most monumental days in astronomical history. That day, the Laser Interferometer Gravitational-Wave Observatory (LIGO) operating in Livingston, LA, and Hanford, WA, measured the “chirp” of two compact objects, in this case, two black holes, merging for the first time, ushering in the era of gravitational wave astronomy. Since then, the LIGO–Virgo–KAGRA (LVK) collaboration, a growing group of partnered worldwide detectors, has observed and publically released 90 such mergers through the end of its O3 run in March 2020 and has observed many more in the ongoing O4 run, which commenced in May 2023. Even more impressive than the first groundbreaking gravitational wave observation is that the detections of mergers are now so routine; potential detections are now recorded on average every few days. One irony of the LVK’s incredible success today is that so many mergers are being observed and identified that it now takes an unlikely or extraordinary observation to catch the attention of scientists and the media. 

Since gravitational wave signals are now observed so regularly, the ones that garner extra attention are those with astrophysical significance. This means that in addition to being rarer events, they can further our understanding of astrophysics in ways that most regular gravitational wave observations alone cannot. One example is a multimessenger event, such as GW170817, which was observed electromagnetically and gravitationally. Another example is the observation of a black hole with mass above what we believe a stellar-mass black hole alone can be under normal conditions. These more massive black holes are referred to as intermediate-mass black holes (IMBHs), and GW190521 is a well-known example. In addition to multimessenger and high mass observations, another special mass range exists on the low-mass end where observations have been rare. If an object were found in this mass range, it would certainly have enough astrophysical significance to be considered one of these extraordinary events.

Enter GW230529

GW230529 is one of those extraordinary events, observed near the beginning of the O4 observing run on May 29th, 2023, and announced by the LVK collaboration about a year later on April 5th, 2024. GW230529 is remarkable for multiple reasons, foremost among them that both objects were low-mass objects, at least on the mass scales the LVK detectors measure. The more massive (or primary) object was estimated to be about 2.5 to 4.5 solar masses, while the less massive (or secondary) object was estimated to be about 1.2 and 2.0 solar masses. 

Figure 1: estimated values for the primary mass (m₁), secondary mass (m₂), and effective spin (ꭓ_eff) for GW230529. Figure 7 in the paper. The three plots along the main diagonal (from top left to bottom right) show the 1-dimensional histograms for each of the three properties. The three off-diagonal plots show the 2-dimensional joint estimate for the two parameters on the associated x and y axes (for example, the primary mass and effective spin for the bottom left plot). Each color/dot type represents a different set of modeling assumptions, with dark blue/solid line being the current default for LVK analyses. You can see the estimates for the masses of the objects by looking at the distributions of dark blue/solid lines in the top left (m₁) and middle (m₂) plots. 

Minding the (Mass) Gap

The mass of the primary object is fascinating because of other evidence for a mass gap in compact objects between about 3 and 5 solar masses. Other observations of neutron stars peak in mass at around 3 solar masses. In contrast, other observations of black holes bottom out at around 5 solar masses, leading to a mass region with few objects in between. It is important to note, though, that the existence and nature of this mass gap are still an open debate among astrophysicists. The LVK observations of GW230529 led the data analysis team to conclude the less massive object was a neutron star. In contrast, they concluded the more massive object was likely a black hole, but with enough uncertainty for the authors to not conclusively claim it as such. Regardless, an object’s existence in the mass gap is a big deal for astrophysics. Another gravitational wave observation – GW190814 – had a component in the mass gap, but in that case, the secondary mass was the one in the mass gap, while GW230529 shows the primary mass as the one in the mass gap. The observation of GW230529 shifts the conversation from whether objects exist in the mass gap to how objects exist in the mass gap and opens new possibilities for understanding how these binary systems with an object in the mass gap may evolve.

Updated rates

Because objects of this mass seem to be quite rare, an additional observation of a single object provides a wealth of information, including how commonplace those objects might be. One analysis the LVK authors conducted was an updated calculation of the merger rate of binary black hole-neutron star systems with the inclusion of the GW230529 event. The merger rate tells us how often we think mergers happen in the universe, usually expressed as the number of mergers for a given (primary) mass in a given volume over a set amount of time. If we know the merger rate from observations, it can be used to constrain theories about how the stars in the universe evolve, become black holes or neutron stars, find each other, and merge to match the observed rate. 

Figure 2: Estimated merger rate of neutron star-black hole systems. Figure 4 in the paper.  The x-axis represents the mass of the black hole in the neutron star-black hole system (by default, the more massive of the two objects). The y-axis represents how many black hole-neutron star mergers are expected for systems with a black hole of that mass, in units of mergers per gigaparsec cubed per year. The gray solid line represents the estimate before including GW230529, and the green solid line represents the inclusion of GW230529. The gray and green regions represent the 90% credible interval regions around the lines. The vertical gray and vertical green dotted lines represent the previous (gray) and current (green) 90% confidence estimates for the minimum mass of a black hole.

Figure 2 shows how adding the single data point of GW230529 changes the expected merger rate for low-mass sources, increasing the number of objects we expect to merge in the range of 2 – 6 solar masses.

Livingston for the win

A final astounding feat for the LVK team was that the entire GW230529 event was analyzed only from data from the Livingston, LA, detector! At the time of the detection, LIGO Hanford was offline, Virgo was undergoing upgrades, and KAGRA was observing but did not have the sensitivity to contribute to the analysis of GW230529. Usually, detections are made when a single gravitational wave event is observed in multiple detectors, vastly increasing the confidence that what was observed was an actual event in all the detectors and not a chance event (e.g. due to noise) in any one of the detectors. It is a testament to the LVK’s experience and data analysis tools that issues like glitches and detector noise can now be ruled out in a single detector confidently enough to claim a detection. The LVK can now do this because it has had enough practice with prior events, and the software tools they’ve developed to analyze gravitational wave data and separate real signals from everything else have advanced considerably in the past few years since the first detections. 

The observation of GW230529 shows how observations by the LVK detectors continue to inform our fundamental astrophysical knowledge about black holes, neutron stars, and the stellar systems that produce them. One of the most amazing things about the gravitational wave field is that, though it has advanced so much in just a few short years, it is only beginning. 

Astrobite edited by Megan Masterson

Featured Image Credit: NASA